A twin screen paper forming apparatus with preliminary draining is shown in German Patent No. DE-OA-3123132. The leading forming roller is positioned above the upper screen and functions as an open forming roller. Water is driven obliquely upwards, but no driving force for producing secondary water streams in the region of the draining rails is produced since these streams run approximately parallel and have substantially uniform speed. A further forming roller is located below the lower screen downstream of the leading forming roller. Draining rails are located opposite the further forming roller so that they do not contact the upper screen.
Other twin screen paper formers have an upper leading forming roller near the upper screen and draining rails in contact with the leading forming roller. These devices have suction chambers disposed between the draining rails for extracting water laterally from the device.